1. Field of the Invention
The present invention relates to a structure of a solar cell and a method of fabricating the solar cell, and more particularly, it relates to an improvement in adhering a solar cell body and a cover glass.
2. Description of the Background Arts
FIG. 4 is a perspective view of a conventional solar cell, such as a GaAs solar cell. The solar cell has a solar cell body 1 in which a photoelectric element for converting a solar light to electric energy is formed, and a comb-shaped electrode 5 is provided on the solar cell body 1. An interconnector 4 for interconnecting solar cells is provided such that an end part of the interconnector 4 is on the electrode 5. The interconnector 4 extends to the exterior of the solar cell and is connected to another solar cell.
In order to prevent the solar cell from being damaged by radiation, a cover glass 2 is adhered on the solar cell body 1 through an adhesive layer 3. The adhesive layer 3 is formed by applying an adhesive material on the solar cell body 1 and hardening the same after the cover glass 2 is placed thereon.
When the solar cell is employed in an artificial satellite, the solar cell is subjected to a temporary change whose range is considerably wide, e.g., from -196.degree. C. to +140.degree. C. Therefore, the solar cell should maintain its desired characteristics in the wide range of the temperature.
However, the adhesive layer 3 sometimes peels from the solar cell body 1 and/or the cover glass 2 due to a stress which is caused from the difference between the respective thermal expansion coefficients of the adhered layers. When the adhesive layer 3 has fully peeled, the solar cell is seriously damaged. Even if the peeling of the adhesive layer 3 is only partial, an interfacial void is generated between the adhesive layer 3 and the layer adhered thereto, so that the light received by the solar cell body 1 is increased, thereby reducing the output power of the solar cell.
The peeling probability depends on the thickness of the adhesive layer 3. FIG. 5 is a histogram showing the relationship between the peeling frequency and the thickness of the adhesive layer 3 after a heating cycle is applied to solar cells, where sixty-one of the adhesive layers 3 were subjected to the heating cycle. In FIG. 5, the void bar VB indicates the number of solar cells subjected to the heating cycle, while the bar BC with cross hatching indicates the number of solar cells in which the adhesive layer peeled. It is understood from FIG. 5 that the frequency of probability of the peeling decreases as the thickness of the adhesive layer is increased. For example, the ratio of the number of peeled adhesive layers to that of the total number of prepared adhesive layers is 9/10 for the thickness range 50-60 .mu.m, while the ratio is zero for thicknesses larger than 90 .mu.m.
However, when a thick adhesive material is applied on the solar cell body 1 and a relatively thin cover glass is placed on the adhesive material, the thickness uniformity of the adhesive material is lost due to interfacial tension between the cover glass and the adhesive material, so that the thickness in the center region of the adhesive material becomes smaller than that in the pheripheral region. Further, if the cover glass 2 is not a flat plate but a curved or cambered plate, the uniformity is partially lost. Furthermore, when the cover glass 2 is inclined from the upper surface of the solar cell body 1, the thickness of the adhesive material or the adhesive layer 3 loses uniformity.